docs: def _extract_mask_distance(image, mask = slice(None), voxelspacing = None):
      """
      Internal, single-image version of `mask_distance`.
      """
      if isinstance(mask, slice):
          mask = numpy.ones(image.shape, numpy.bool)
      
      distance_map = distance_transform_edt(mask, sampling=voxelspacing)
      
      return _extract_intensities(distance_map, mask)